1. Field of the Invention
The invention relates to an overvoltage protection device for protecting the electrical system of a vehicle against overvoltages in particular in a vehicle having an on-board wiring network, fed by an electrical generator.
2. Background Information
Motor vehicles usually have an electrical system comprising a multiplicity of electrical devices fed from an on-board network. The on-board wiring network in turn is fed by an electrical generator. Furthermore, the on-board network has a battery connected thereto from which the on-board network is fed when the generator delivers too little electrical energy or no electrical energy at all. The latter is the case when the engine of the vehicle is at a standstill.
During operation, overvoltages may occur in the on-board wiring network, for instance in case of temporary interruptions between the battery and the on-board network, which may be caused by disconnection of the battery while the engine, and thus the generator, is running or may be due to intermittent or loose contacts. Overvoltages may also occur in case of so-called load dumps, i.e. when electrical equipment of the vehicle is switched off during operation, such as blower motors, setting motors for seat adjustments, setting motors for power windows, etc. As a result of such load dumps, a reduction of the magnetic field present in the generator occurs and, as is generally known, such reduction leads to inductively generated voltage pulses that may reach considerably high voltage and energy values. The pulse height is dependent on the field excitation of the generator, on the speed thereof and on the load remaining at the generator at the moment of disconnection from the battery or at the moment of load dump.
More detailed information in this respect is available from DIN (German Industrial Standard) 40 839, Part 1, in particular section 4.6.5, ISO/TR 7637/1 to section 2.3.2.5, and from a publication of the automotive supply company Bosch entitled "Uberspannungsschutz" (overvoltage protection).
Such voltage spikes cause problems in the electrical systems of modern motor vehicles. Quite a lot of the electrical equipment of such vehicles, e.g. on-board computers, electronic control devices for anti-lock systems, the electronic control system for the internal combustion engine, include integrated semiconductor circuits which may be caused to malfunction due to overvoltages, or be permanently damaged thereby.
Various attempts have been made to control such harmful interference voltage pulses. One possibility uses so-called suppressor diodes disposed either in each of the electrical devices of the electrical system of the vehicle or centrally at the generator. Such diodes are supposed to limit the interference voltage pulses to a safe maximum voltage. However, this solution entails the following problems.
A substantial part of the energy during load dump is converted to heat in a suppressor diode. Calculations show that the temperature in the suppressor diode may be increased by approx. 90.degree. C. for example. At ambient temperatures of about 100.degree. C., as may occur in the engine compartment of motor vehicles, chip temperatures of 190.degree. C. and thus created. These temperatures are above the temperatures usually managable for common semiconductor components.
Furthermore, there is the fact that the voltage limited by the suppressor diode is strongly dependent on the current and the voltage tolerance range to be taken into consideration is thus increased considerably. With suppressor diodes, considering the current dependency thereof, the tolerance range of the voltage to be limited is between 24 V minimum and 40 V maximum. This leads to considerable problems in the on-board wiring network, since further protective diodes for limiting lower energy pulses must be provided above this toleranace range. Thus, the on-board network is afflicted with positive energy pulses of up to 50 V, despite a considerable expenditure of protective diodes.
Due to these voltage spikes, the semiconductor components used in the electronic devices and modules must have a breakdown voltage V.sub.CE0 of .gtoreq.50 V. This results in considerable additional costs since high voltage technologies must be employed and the required chip area is increased thereby.
It is known from the afore-mentioned publication "Uberspannungsschutz" to avoid overvoltages by short-circuiting the generator with the aid of a thyristor when the generator voltage exceeds a specific overvoltage threshold. In this case, the thyristor is activated via the gate thereof, whereby the thyristor changes to a condition of low impedance which virtually constitutes a short-circuit for the generator. However, this short-circuit condition can be terminated only by opening the ignition lock of the vehicle in order to thereby interrupt the voltage suppply of the overvoltage protection circuit and thus of the thyristor.
This problem is overcome by means of an overvoltage protection circuit as it is known from the publication "Uberspannungsschutz" as well, which comprises an automatic activation means, which cancels the short-circuit of the generator when the interference voltage is over. For this purpose the thyristor has a relay connected in series therewith which, in case of activation of the thyristor, has the generator short-circuiting current flowing therethrough and as a consequence thereof constitutes a path bridging the anode-cathode path of the thyristor. On the one hand, this short-circuits the anode-cathode path of the thyristor so that the latter can switch off. On the other hand, the generator is held in the short-circuited condition also after switching-off of the thyristor. The relay will reopen the switch only after the generator current in the relay winding has dropped to a specific value.
The relay of this known overvoltage protection circuit entails problems. On the one hand, relays as electromechanical components are subject to mechanical wear and thus have a correspondingly restricted lifetime.
On the other hand, relays lead to tolerance problems as a result of aging effects, production leakages etc. In order to safely protect the sensitive semiconductor components of the electrical system of the vehicle it is thus necessary to again allow for a relatively large voltage range and to design the semiconductor components again for relatively high voltage strengths. Furthermore, besides, due to their inductive properties, relays react in a comparatively slow manner to electrical changes.